"There is a time to learn, a time to lead and a time to leave," he said in a video statement on his union's Web site. "There is never a perfect time for any long-term leader to depart or for organizations to make transitions."

Stern spent 14 years heading the 2.2-million member union that he helped build into one of the nation's largest and most politically active. He said he realized a dream with the passage of health care overhaul legislation and that his departure would pave the way for new leadership at SEIU.

But his decision to leave the post two years before his term ends stunned many in the labor and political worlds. Stern had an especially high profile with Democratic control of the White House and Congress. He is a close confidant of President Barack Obama and his fingerprints are on many of the administration's policies.

Stern did not say exactly when he would leave, but indicated it would be soon. Secretary-Treasurer Anna Burger will replace Stern for 30 days until the board elects a new president.

One longtime labor observer said Stern may have become overwhelmed by all the internal labor squabbles and legal fights left in the wake of his hard-driving persona.

"This is not going out on top at all," said Nelson Lichtenstein, a labor historian at the University of California, Santa Barbara. "He faced all these internal issues and they began to create real problems for him."

Among those, a bitter battle for control of one of his union's largest locals in Northern California. Labor Department filings show the SEIU spent millions in legal bills last year to battle a breakaway group that formed a rival West Coast union.

Stern also was criticized by other union leaders for inserting himself into a feud between factions at the union of hotel, restaurant and clothing workers known as UNITE HERE.

Despite the controversies, University of California-Berkeley labor studies professor Harley Shaiken said history would view Stern as "a defining figure in American labor."

"His legacy is effective organizing and he went after those who were traditionally viewed as unorganizable, like home health care workers and janitors," Shaiken said. "He organized the dispossessed."

Stern doubled the size of the SEIU, raising membership by more than 1.2 million.

Stern used his union's money and organizing power to further his liberal agenda, whether it was overhauling health care, seeking changes to immigration policy or campaigning for workers to earn a living wage.

In 2005, he led his union and six others to break away from the AFL-CIO — the nation's largest labor federation — to form the rival Change to Win federation. He said the older federation was not effectively organizing new members to replace the millions dropping from union ranks year after year.

The move ended up splitting the labor movement. Critics said the new federation was ineffective and sapped the strength of a unified union movement.

Stern did not say what he would do next, though officials said he planned to keep his position as a member of Obama's deficit commission.

He talked about the loss of his daughter, who died in 2002.

"The loss of my 13-year-old daughter Cassie and the 24/7 responsibilities of this job have left me at times not paying enough attention to the personal dimensions of my life," he said. "I do not have another job or any specific plans for now."